OPP: How Do I Tell My Brother He Is Making A Mistake?

I listen ever day in my car on the way to work and wanted to see about getting your advice. My Brother and I for the last few years have had an up and down relationship. The stress it's caused our family was getting pretty bad. We have gone years with out talking to each other. We decided to put our differences to the side and get the entire family together over the long Memorial Day weekend.

While I may not be the best sister ever, I do love my brother and want the best for him. He just got engaged to a girl who I met for the first time over the weekend. There is just something off about her. I can't really put my finger on it. After spending some time with her and talking to her it sounds to me like she is really not in to him as much as he is to her.While we all were together I ended up walking up behind her and saw her swiping through Tinder. I said something to her about it as a joke and she said it makes her feel like she still "HAS IT". I told her that if she is engaged shouldn't she be done with those apps and that kind of world. She told me that it doesn't matter "Your brother wouldn't care." I guess I am shocked. My brother and I are just getting to a place were we are working on our relationship and the last thing I want to do is make it bad again. What should I do? I feel like as his older sister I should say something to him. I'm just so worried about going backwards.